Working vehicles such as wheel loaders are also subjected to emission control, and working vehicles with engines mounted thereon and restricted in fuel injection volume to meet emission control are conventionally known. Emission control compliant engines lack so-called tenacity, and are liable to stall when subjected to a large load. Especially in a working vehicle of the torque converter drive system like a wheel loader of the torque converter drive system that an output torque from an engine is transmitted to a transmission via a torque converter to generate traveling drive force, the engine is liable to stall because for its work characteristics, a high load tends to act on the engine even in a state that an accelerator is not depressed.
When the accelerator pedal is released while performing hill-climbing travel, for example, under forward travel selected by a forward/reverse command means, the traveling drive force decreases so that the working vehicle may roll down by its own weight. As a manner of operation of such a wheel loader, it is often practiced to operate the forward/reverse command means during forward travel for the selection of reverse travel or to operate the forward/reverse command means during reverse travel for the selection of forward travel, and then to control the depression stroke of the accelerator pedal for the adjustment of the braking force without operation of a brake pedal. In such a situation, a load by its own weight or an inertia force acts on the engine via tires, the transmission and the torque converter so that upon release of the accelerator pedal, the output torque of the engine becomes lower than the load acting on the engine, leading to engine stall in a worst-case scenario.
In the working vehicle of the torque converter drive system, a steering mechanism and front working equipment, such as a bucket, are driven by the energy of pressure oil delivered from a hydraulic pump driven by the engine. Operation of the steering mechanism and drive of the front working equipment, therefore, apply loads on the engine. When high hydraulic load work that “lifts the bucket while performing steering control” is performed with the target revolution speed for the engine having been set at a low idle speed, for example, the engine becomes more liable to stall. Such a problem becomes particularly pronounced in a working vehicle with an emission control compliant engine mounted thereon and restricted in fuel injection volume.
As a stall prevention technology for a vehicle-mounted engine, a technology has been proposed to determine whether a working vehicle is in a rolling down state or in a non-rolling down state and, when the working vehicle is in the rolling down state, to automatically increase the engine speed than that when the engine is in the non-rolling downstate (see, for example, Patent Document 1). Another technology has also been proposed to detect a speed of an engine and, when the speed of the engine is determined to have dropped to a predetermined threshold level or further, to automatically reduce a torque to be absorbed by a variable displacement hydraulic pump driven by the engine (see, for example, Patent Document 2).